Maximum power can only be obtained from an internal combustion engine if the ignition timing is correct. Any timing fluctuation between cylinders (spark scatter) or timing fluctuation at a single cylinder (spark jitter) reduces the power output of the engine. Common causes of timing fluctuation are the flexing and end play of the engine camshaft; distributor gas "play"; distributor point cam inaccuracies; distributor point bounce and wear; timing chain and gear wear; and false ignition triggering. These inaccuracies are particularly important in high performance, high RPM engines, because these engines normally utilize the maximum advance possible before pre-ignition occurs. Precise ignition timing allows this advance limit to be more closely approached.
Ordinary prior distributors commonly exhibit timing inaccuracies of several degrees, particularly at high RPM. Point bounce has been eliminated through the use of magnetic pulse distributors and the points cam has been eliminated, but the remaining causes of ignition timing inaccuracies are still present and create significant timing errors.
An effort to solve these problems and produce precision timing pulses haa been undertaken by several manufacturers. The solution is general has been the substitution or addition of a crankshaft trigger wheel. The wheel is usually attached to the engine crankshaft directly by bolting it to the engine's harmonic balancer. The wheel of the prior art is usually made of aluminum with four ferrous steel tabs located 90.degree. apart (for an eight cylinder engine) on the wheel outer perimeter. Others have employed ferrous steel with teeth or notches radially spaced around the perimeter. A magnetic pickup is located in a selected fixed position in close proximity to the tabs, teeth, or notches so that a suitable electrical pulse is generated in the magnetic pickup as the tab passes near the pickup during crankshaft rotation. The resultant pulses are used to trigger an electronic ignition system which in turn produces the high voltage ignition pulses used to ignite the combustion mixture. The prior art crank trigger wheels provide a substantially higher degree of timing accuracy than conventional distributor system; however, they still possess several negative characteristics. The most serious of these is the lack of sufficient noise immunity in order to prevent false ignition triggering, a major cause of drastic ignition timing inaccuracy. Another shortcoming of crankshaft trigger wheels of the prior art is the two piece construction utilized with aluminum wheels. These wheels use steel tabs or pins as stated above which can loosen and therefore create timing jitter or become completely separated from the wheel.